Various systems have been devised for drilling and completing two or more offshore wells through a conductor pipe which extends upward to a single wellhead. U.S. Pat. No. 5,458,199 discloses an assembly for drilling and completing multiple subterranean wells from a common wellbore. A single riser with two wellheads is disclosed in U.S. Pat. No. 5,810,086. Techniques for completing and cementing a juncture with lateral wellbores are disclosed in U.S. Pat. No. 5,477,925. More recently, U.S. Pat. No. 6,279,658 discloses a method of forming two wellbores from a single wellbore. Other relevant patents include U.S. Pat. Nos. 5,330,007, 4,807,704, 4,742,871, 4,640,353, 4,573,541, 4,415,205, 4,396,230, 4,396,075 and 4,068,729.
For offshore drilling and completing operations, a conductor pipe is conventionally jetted or otherwise driven into the sea floor. From within this conductor, the operator sequentially drills two or more holes of production casing strings. The driller needs to ensure that the drilled holes do not overlap, breaking into each other's bore. To prevent overlap, various styles of splitters or guide assemblies have been devised.
One solution is to provide a separator curtain from the surface, with the curtain effectively forming a wall with intermittent struts, tubes or other structural shapes acting as guides. The separator curtain may resemble a single wall across the center of the pipe, or one may provide an alternative X configuration. Providing a single wall across the pipe center may result in north-south separation, but no control in the east or west direction, allowing the drill string to wander before the conductor wall and the separator curtain prohibit sideways movement. The X profile is thus preferred for many applications, since it provides a more definite and precise separation in north, south, east and west directions. These designs may maintain an open bore during the drilling operation and then subsequently are installed to provide separation and guidance of the drill string. The separator curtain design is, however, heavy and awkward, and expensive and time consuming to deploy.
An alternative splitter assembly may utilize one or more conductor joints, with a web or portion of a separator curtain welded in place within the conductor joints. Manufacturing and deploying expenses are reduced, and this separator desirably serves its purpose near the bottom of the conductor. The partial blocking of the conductor bore by the webbed, or other partial separator curtain, results in a significant drag during the operation of driving the conductor joints into the sea floor.
A further alternative directly drives an open bore conductor into a seabed, then runs a sleeve assembly through the conductor down to the shoe at the lower end of the conductor. The sleeve may serve as a guide to ensure that the sequentially drilled holes maintain separation, and may thereafter diverge slightly outward. The conductor bore which supports the sleeve provides a continuous support shoulder, which undesirably blocks off the bore, making driving to the desired depth more difficult. The sleeve assembly is also complex, expensive to manufacture, and utilizes a rather complex cycling procedure to index for proper alignment.
Yet other splitters or downhole guide assemblies have been proposed which do not rotate or index over the proper bore, and instead rely on axially staggered spacing between a primary and a secondary bore. The operator may tally string length and thereby estimate which staggered height bore has been entered. A staggered length between the primary and secondary bores is thus desirable for the operator, but also increases the length of the assembly, which is highly undesirable.
The disadvantages of the prior art are overcome by the present invention, and an improved downhole splitter or guide assembly and method are hereinafter disclosed.